This invention relates generally to swashplate type axial-piston hydraulic pumps, and in particular to an improved bearing arrangement for rotatably supporting the rotatable cylinder barrel in such a pump.
Swashplate type axial-piston hydraulic pumps are well known in the art and typically include a generally cylindrical cylinder barrel rotatably mounted within a pump housing. One or more pump cylinder bores, having pump pistons reciprocably mounted therein, are disposed around the rotational axis of the cylinder barrel in parallel, or almost parallel alignment therewith. The ends of the pistons project beyond the end of the cylinder barrel so as to engage the surface of an angled swashplate stationarily mounted adjacent the end of the cylinder barrel within the barrel housing. When the cylinder barrel is rotated within the housing, slipperpads, mounted to the piston ends, follow the surface of the angled swashplate with the result that the pistons are reciprocated within their respective cylinder bores. A fluid control valve assembly, disposed adjacent the end of the cylinder barrel furthest from the swashplate, controls the ingress and egress of hydraulic fluid from the piston cylinders such that a pumping effect is produced in response to rotation of the cylinder barrel within the pump housing.
Since the plane of the swashplate is inclined relative to the rotational axis of the cylinder barrel, the contact force between each piston head slipperpad and the swashplate includes a substantial, non-zero, radial component in addition to an axial component which actually drives the pistons. This radial component tends to cause cocking or pitching of the cylinder barrel within the pump housing. As proper operation of the pump depends on a very close fitting relationship between the inlet and outlet port of the cylinder barrel and the fluid control valve assembly, any pitching or cocking of the cylinder barrel affects the operation of the pump and may result in damage or excessive wear of the valve assembly.
The resultant sum of all the contact forces exerted on the cylinder barrel by each piston can be resolved into single equivalent axial and radial force components operating on a single equivalent force point located along the rotational axis of the cylinder barrel and displaced from the end thereof in a direction toward the swashplate. Preferably, the cylinder barrel is journaled within the pump housing such that the radial component of the resultant equivalent force is balanced by the bearing forces. When so journaled, cylinder barrel cocking and pitching can be reduced or eliminated.
One well known design which sought to balance the radial force on the cylinder barrel of a swashplate type hydraulic pump included an elongate extension collar on the exterior circumference of the cylinder barrel. The collar extended beyond the end of the barrel and was of sufficient length to encircle the equivalent force point located near the swashplate. A roller or sleeve bearing was located between the exterior of the extension collar and pump housing. Even though the equivalent force point was spaced away from the end of the actual end of the cylinder barrel, the extension collar nevertheless permitted the bearing to be located in a plane which was perpendicular to the rotational axis of the cylinder barrel and which intersected the equivalent force point. When so located, the bearing opposed the radial force component and pitching of the cylinder barrel was minimized. While this prior design was effective in reducing cylinder barrel pitching, the need for the cylinder barrel extension collar to encircle the actual equivalent force point required that the collar also encircle the swashplate and slipperpads of the piston heads supported therefrom. This increased the axial and radial dimensions of the pump and resulted in increased complexity, cost and internal operating friction.
The present invention is directed to an improved swashplate type axial-piston hydraulic pump wherein the rotating cylinder barrel of the pump is supported against the radial force components by a bearing spaced away from the equivalent force point in a direction along the cylinder barrel rotational axis such that little or no pitching moments are created a the cylinder barrel rotates. This is accomplished without the use of an extension collar and without the need for a bearing to encircle the swashplate and piston head slipperpads.
In accordance with one principal aspect of the invention, the bearing between the cylinder barrel and the pump housing is located between the ends of the cylinder barrel and, accordingly, is displaced substantially from the equivalent force point in a direction along the rotational axis of the cylinder barrel. While the plane of the bearing is axially displaced from the equivalent force point, the bearing is arranged such that all bearing forces which are developed perpendicularly to the bearing race define an acute angle relative to the rotational axis of the cylinder barrel and intersect the rotational axis in the general area of the equivalent force point. Thus, the location and orientation of the bearing is such that the support forces developed by the bearing generally define a cone around the cylinder barrel rotational axis. The base of the cone so defined is defined by the bearing race and the apex of the cone lies on the cylinder barrel rotational axis at the equivalent force point. The height of the cone is thus equal to the distance by which the bearing is displaced from the equivalent force point. This distance allows the effective support of the cylinder barrel to be moved axially away from the swashplate in the direction of the cylinder barrel so that the cylinder barrel bearing can be located in front of the swashplate and piston head slipperpads. Thus, the need for an extension collar is eliminated since the bearing can now be located directly on the cylinder barrel. This permits smaller radial and axial dimensions in the completed pum and further results in reduced cost and operating friction.
As the cylinder barrel rotates, there is a small cyclical change in the actual location of the equivalent force point along the cylinder barrel rotational axis. Accordingly, since the apex of the con formed by the resulting bearing force is stationary on the rotational axis, the equivalent force point on which the radial force acts, and the point on which the bearing forces act, will coincide exactly only momentarily. However, the distance over which the equivalent force point moves during rotation of the cylinder barrel is very small and the resultant pitching moment of the barrel is so small as to be negligible.
In order to support the cylinder barrel against the axial components of the contact force between the swashplate and the piston head slipperpads, the bearing also develops substantial axial force components in addition to the radial force components. The axial components are easily obtained since the bearing, in most practical applications, is located considerably in front of the equivalent force point. Thus, the inclined resultant of the bearing forces will generally include a substantial axial component.
In accordance with another principal aspect of the invention, the race of the bearing which rotatably supports the cylinder barrel is of sufficient width so that the resultant bearing forces do not converge to a point on the rotational axis of the cylinder barrel but rather define a line segment, equal in length to the width of the bearing race, along the rotational axis. Preferably, the width of the bearing race, and accordingly, the length of this line segment, is sufficiently great so that the equivalent force point of the swashplate contact forces remain located within the projection of the bearing race during the cyclical movement of the equivalent force point along the rotational axis.
In still another principal aspect of the present invention, the bearing is located directly between the outer circumference of the cylinder barrel and the sidewall of the pump housing. This results in a large angle between the bearing forces and the cylinder barrel rotational axis which is advantageous.
In still another principal aspect of the present invention, the pump housing includes a journal disposed coaxially with the rotational axis of the cylinder barrel. A bearing carried on the journal engages the cylinder barrel to support the cylinder barrel for rotation within the pump housing. In this arrangement, the bearing does not extend beyond the outer circumference of the cylinder barrel. Additionally, the journal can be located either on the side of the cylinder barrel nearest the swashplate, or, on the side of the cylinder barrel opposite the swashplate. In all cases, either hydrodynamic or hydrostatic slide or roller type bearings can be advantageously employed.